THE PLASMA DEPLETION LAYER BEYOND THE HELIOPAUSE: EVIDENCE, IMPLICATIONS, AND PREDICTIONS FOR VOYAGER 2 AND NEW HORIZONS

Abstract

ABSTRACT A plasma depletion layer (PDL) is predicted beyond the heliopause, analogous to the PDLs observed sunwards of planetary magnetopauses: draping of interstellar medium (ISM) magnetic field lines over the heliopause should increase the magnetic field strength and, perpendicular ion temperature, cause density depletions by allowing plasma ions (and electrons) with large parallel temperatures to escape along , and increase the temperature anisotropy until limited by wave instabilities. Published Voyager 1 magnetometer and plasma wave data provide strong evidence for the coupled magnetic amplification ( ) and density depletion ( ) expected for a weak PDL. The predicted reduction in parallel temperature is . The locations on the sky of the PDL and the points on the heliopause of maximum magnetic draping and total pressure are predicted using the ISM magnetic field direction obtained from the Interstellar Boundary Explorer (IBEX) ribbon. The IBEX ribbon overlies the former, as expected, while the latter lies within the ridge of maximum, non-ribbon, globally distributed flux. The PDL should be strongest along the ISM field line passing through these points and the Sun–ISM velocity vector. Based on their trajectories, Voyager 2 and New Horizons should observe a much stronger PDL (stronger magnetic amplification, density depletion, and changes in temperature) than Voyager 1. Finally, the reduced cosmic ray fluxes observed near 90° pitchangle by Voyager 1 beyond the heliopause appear qualitatively consistent with wave–particle interactions transferring perpendicular particle energy to parallel energy where the PDL is strong, followed by magnetic focusing as particles propagate into weaker magnetic field regions.